FIG. 1 shows a conventional user plane protocol stack 100 for the Long Term Evolution (LTE) architecture. The stack 100 includes a Packet Data Convergence Protocol (PDCP) layer, Radio Link Control (RLC) layer, Medium Access Control (MAC) layer, and physical (PHY) layer in a wireless transmit receive unit (WTRU) and corresponding layers in an evolved Node-B (eNB). The eNB is connected to a System Architecture Evolution (SAE) gateway by means of an S1-U interface.
According to the Third Generation Partnership Project (3GPP) standard, the LTE MAC sublayer supports mapping between logical channels and transport channels. The MAC sublayer supports multiplexing of MAC service data units (SDUs) from one or more logical channels onto transport blocks (TBs) for delivery to the PHY layer via transport channels. The MAC sublayer also supports demultiplexing of the MAC from one or more logical channels from the TBs delivered from the PHY layer via transport channels. In addition, the MAC sublayer supports logical channel prioritization and transport format selection.
A function of the MAC sublayer is the prioritization of the logical channels. The MAC entity may receive the MAC SDUs from different logical channels that are from the RLC layer. The MAC entity then multiplexes the SDUs onto one transport channel.
FIG. 2 shows the MAC multiplexing data from multiple logical channels onto one transport channel. The multiple logical channels may include dedicated traffic channels (DTCHs), dedicated control channels (DCCHs), or common control channels (CCCHs). In the example of FIG. 2, the one transport channel is shown as an uplink shared channel (UL-SCH).
The logical channel prioritization is applied when a new MAC transmission is performed. A radio resource control (RRC) sublayer in the WTRU controls scheduling of uplink data by assigning each logical channel a priority value. An increasing priority value indicates a lower priority level for the logical channel. Additionally, each logical channel is assigned a prioritized bit rate (PBR) and a maximum bit rate (MBR). A WTRU serves all logical channels in a decreasing priority order up to their configured PBR. If any resources remain, all logical channels are served in a decreasing priority order up to their configured MBR. If a MBR is not configured, then the logical channels are served until the data for the logical channel is exhausted or an uplink grant is exhausted, whichever comes first. The WTRU serves all logical channels configured with the same priority equally. The MAC control elements for buffer status report (BSR), with the exception of padding BSR, have higher priority than user plane logical channels.
In the 3GPP standards, a WTRU has an uplink rate control function that manages sharing of uplink resources between radio bearers. The RRC controls the uplink rate control function by giving each bearer a priority and a PBR. The RRC also provides an MBR per guaranteed bit rate (GBR) bearer. The values signaled by the WTRU may not be related to the values signaled via interface S1 to the eNB. If more than one radio bearer has the same priority, then the WTRU serves these radio bearers equally.
There are several proposals discussing the details of logical channel prioritization and MAC multiplexing. The proposals agree on specifying the input parameters and the constraints for the output of the WTRU.
The proposals assume a token bucket model for the specification of input parameters. The PBR or the MBR is derived from the WTRU using the token rate, from a fixed size, or is signaled from the eNB. The PBR or the GBR does not limit the reported buffer status. The proposals utilize a token bucket model to describe the rate calculations, whereby each logical channel may have token buckets, related to the PBR and the MBR. The rates at which tokens are added to the buckets are the PBR and the MBR. A size of the token bucket may not exceed a predetermined maximum value.
One proposal describes a process for rate calculations or token bucket calculations. For each time increment of a bearer, Tj, that has a PBR, the PBR credit associated with the bearer j is incremented by a value of Tj×PBRj. If the bearer also has an MBR, then the MBR credit associated with the bearer j is incremented by a value of Tj×MBRj. If upper limits are set for the maximum PBR and if the accumulated values exceed the maximum values, then they are set to the maximum value. If the MBR credits are set for the bearer j, and if the accumulated values exceed the maximum values, then they are set to the maximum value. At each scheduling opportunity transmission time interval (TTI), where the WTRU is permitted to transmit a new data, the scheduler selects data from the highest priority bearer that has a non-empty buffer state and non-zero PBR credit. The scheduler may add to the TB data equal to the size of the buffer, the size of the PBR credit, or the available capacity of the TB, whichever is the smaller. The PBR credit and the MBR credit are decremented by the quantity of data assigned.
If the PBR credit of all bearers is zero and a space is available in the TB, then the scheduler accepts data from the highest priority bearer with a buffered data, up to the size of the available space in the TB or the MBR credit of the WTRU, whichever is the smaller. The MBR credit is decremented by the quantity of data that was accepted.
FIG. 3 shows a conventional MAC protocol data unit (PDU), which includes a MAC header, MAC control elements, MAC SDUs, and padding bits. Both the MAC header and the MAC SDUs may be of variable size. The MAC PDU header includes at least one MAC PDU sub-header, where each sub-header corresponds to either a MAC SDU, a MAC control element, or padding bits. The MAC layer may generate MAC control elements, such as the BSR control elements. The MAC control elements may be identified via specific values for the Logical Channel ID (LCID), as illustrated in Table 1 below.
TABLE 1IndexLCID values00000-yyyyyIdentity of the logicalchannelyyyyy-11100reserved11101Short Buffer StatusReport11110Long Buffer StatusReport11111Padding
The indexes 00000-yyyyy, shown in Table 1 above, may correspond to actual logical channels that have corresponding RLC entities. The remaining indexes may be used for identifying the MAC control elements, such as BSRs or padding.
According to the 3GPP standards, some services and functions of the LTE RLC sublayer include transfer of upper layer PDUs supporting acknowledged mode (AM) or unacknowledged mode (UM). The RLC sublayer also include transparent mode (TM) data transfer, error correction through automatic repeat request (ARQ), in-sequence delivery of upper layer PDUs (except at handover in the uplink), flow control between the eNB and WTRU, SDU discard, and RLC reset. Accordingly, the RLC supports three modes of operation: the AM, the UM, and the TM.
To ensure service continuity and to minimize service interruption, it would be beneficial to provide a method an apparatus to initialize token buckets, to preserve token buckets upon certain events, to reconfigure token buckets, and to communicate the status of token buckets using enhanced BSRs or new control elements.